The present invention relates to electrochemical fuel cells. In particular, the invention provides a method and apparatus for controlling the temperature within a fuel cell using a heat transfer liquid travelling in the same fluid passages as a reactant fluid.
Electrochemical fuel cells convert reactants, namely fuel and oxidant fluids, to generate electric power and reaction products. Electrochemical fuel cells employ an electrolyte disposed between two electrodes, namely a cathode and an anode. The electrodes each comprise a quantity of electrocatalyst disposed at the interface between the electrolyte and the electrodes to induce the desired electrochemical reactions. The location of the electrocatalyst generally defines the electrochemically active area of the fuel cell.
Solid polymer fuel cells generally employ a membrane electrode assembly (xe2x80x9cMEAxe2x80x9d) which typically consists of a solid polymer electrolyte, or ion exchange membrane, disposed between two electrode layers comprising porous, electrically conductive sheet material. The membrane is typically proton conductive, and acts as a barrier for isolating the fuel and oxidant streams from each other on opposite sides of each MEA. The membrane also substantially electronically insulates the electrodes from each other.
In a fuel cell stack, the MEA is typically interposed between two separator plates that are substantially impermeable to the reactant fluids. The plates act as current collectors and provide support for the adjacent electrodes. To distribute the reactant fluids to the respective electrochemically active area of each electrode, the reactant fluid passages may comprise open-faced channels or grooves formed in the surfaces of the plates that face the MEA. Such channels or grooves define a flow field area that generally corresponds to the adjacent electrochemically active area. Such separator plates, which have reactant channels formed therein are commonly known as flow field plates. The flow field plates, together with the porous electrode layer define reactant fluid passages adjacent both sides of each membrane.
In a fuel cell stack a plurality of fuel cells is connected together, typically in series, to increase the overall output power of the assembly. In such an arrangement, one side of a given plate may serve as an anode plate for one cell and the other side of the plate may serve as the cathode plate for the adjacent cell. To improve the viability of fuel cells as a commercial power source, it is generally desirable to improve the power density of the stack, that is, to reduce the stack dimensions and weight for a given electrical power output capability.
The fuel fluid stream which is supplied to the anode may be a gas such as substantially pure hydrogen or a reformate stream comprising hydrogen. Alternatively, a liquid fuel stream such as, for example, aqueous methanol may be used. The oxidant fluid stream, which is supplied to the cathode, typically comprises oxygen, such as substantially pure oxygen, or a dilute oxygen stream such as air. In a fuel cell stack, the fuel and oxidant fluid streams are typically supplied to respective electrodes by respective supply manifolds and exhausted from the same electrodes by respective exhaust manifolds. Manifold ports fluidly connect the stack manifolds to the flow field area and the electrochemically active area of each fuel cell. (See U.S. Pat. Nos. 5,484,666 and 5,514,487, each of which is hereby incorporated by reference in its entirety, and which disclose examples of fuel cell stack manifold configurations.)
In conventional solid polymer fuel cells incorporating an ion exchange membrane, water is normally used to hydrate the membrane to improve its ionic conductivity. Hydration may also help to maintain the resilience of the membrane, reducing the potential for structural failure, which may occur if the membrane becomes too dry.
Conveniently, water is produced within the MEA as a product of the desired electrochemical reactions at the cathode. However, the quantity of water produced at the cathode is typically insufficient to keep the membrane suitably hydrated, so additional water is often introduced into one or both of the reactant streams, usually as water vapor. In conventional fuel cells, an objective is typically to keep water in the vapor phase in the vicinity of the MEA and to manage the cathode product water so that it evaporates into the cathode reactant stream.
The electrochemical reaction in a solid polymer fuel cell is typically exothermic. Accordingly, a cooling system is typically needed to control the temperature within a fuel cell to prevent overheating. Since a water supply is often present in conventional fuel cell systems for humidification, conventional designs have commonly used water as a coolant (see, for example, U.S. Pat. No. 5,200,278, hereby incorporated by reference herein in its entirety, which discloses a conventional fuel cell power generation system which employs water as a coolant).
However, because the solubility of the typical gaseous reactants in water is low, if water or aqueous liquids are introduced into a reactant fluid stream as a liquid coolant, the presence of the liquid coolant generally reduces the accessibility of the reactants to the electrocatalyst. Conventional fuel cells typically seek to avoid gas-liquid two phase flow within the reactant fluid passages by isolating liquid coolant streams from gaseous reactant streams by employing separate cooling fluid passages which are fluidly isolated from the reactant fluid passages (see for example, FIGS. 1, 2A and 2B in U.S. Pat. No. 5,230,966 and the accompanying description, which is hereby incorporated by reference herein in its entirety).
While conventional arrangements such as the one described in the preceding paragraph can control the temperature within a fuel cell stack, these arrangements generally require separate cooling layers (for example, coolant flow field plates) and manifolds for directing the coolant fluid through the stack. In a conventional fuel cell stack the cooling layers often occupy at least about one third of the plate volume. Therefore, the power density of a conventional fuel cell stack could be improved if the cooling layers could be eliminated. Furthermore, another disadvantage associated with separate cooling plates and a fluidly isolated cooling system is that additional seals and manifolds are required to contain the coolant and to keep the coolant fluidly isolated from the reactants.
Another problem associated with using water or other aqueous coolants in fuel cells is that such coolants may expand upon freezing and damage stack components. Also, frozen water in the active area can be difficult to remove since it must be melted and then purged to allow the reactant to access the catalyst. It is anticipated that fuel cells may be used in vehicles or installations where the fuel cell may be exposed to temperatures below the freezing temperature of water. For example, automobiles are typically designed for exposure to temperatures as low as xe2x88x9240xc2x0 C. Therefore, when water or aqueous liquids are used as liquid coolants, conventional fuel cells intended for such applications need to incorporate features to prevent freeze expansion damage. This is another reason why conventional fuel cells avoid using water or aqueous coolants directly in the reactant stream passages. The porous electrodes and thin membrane are particularly susceptible to freeze expansion damage. As it is, precautions must be taken to deal with product water and reactant stream humidification water if the fuel cell is going to be exposed to freezing conditions.
Accordingly, there is a need for a method of operating a fuel cell, and an apparatus for implementing such a method, which reduces or eliminates some or all of the problems and disadvantages described above.
In the present approach, a method is provided for controlling temperature within an electrochemical fuel cell which comprises an electrolyte interposed between first and second electrodes and a quantity of electrocatalyst disposed at an interface between the electrolyte and each of the first and second electrodes. Such a method comprises:
(a) introducing to the first electrode a reactant fluid stream comprising a reactant and a heat transfer liquid, such that the reactant fluid stream contacts the first electrode;
(b) removing a reactant fluid exhaust stream from the first electrode, the reactant fluid exhaust stream comprising the heat transfer liquid; and
(c) recirculating at least a portion of the heat transfer liquid from the reactant fluid exhaust stream to the first electrode via a heat exchanger, whereby the temperature of the heat transfer liquid is controlled.
Preferably the fuel cell is a solid polymer electrolyte fuel cell and the fuel cell is one of a plurality of fuel cells arranged in a stack. The reactant fluid stream may be pre-mixed, or the method may further comprise the step of combining the reactant with the heat transfer liquid to produce the reactant fluid stream.
The heat transfer liquid is not water, and preferably it is non-aqueous. However, the overall reactant fluid stream may comprise water.
For example, hydration water and reaction product water may be present in the reactant fluid stream. The heat transfer liquid may have other desirable characteristics, such as, for example, being aprotic and/or dielectric, for example, so that the heat transfer liquid does not cause current leakage or short circuiting. It is also preferable for the heat transfer liquid to be chemically unreactive towards other fluids within the reactant fluid stream and/or chemically unreactive towards fuel cell components that directly contact the reactant fluid stream.
Some examples of preferred heat transfer liquids are paraffin oils, fluorocarbons, and hydrocarbons. In particular, if the heat transfer liquid is a fluorocarbon, it may be a perfluorocarbon. Specifically, a preferred heat transfer liquid may be selected from the group consisting of methanol, perfluorooctane, perfluorotributylamine, 1-decene, perfluoroether, perfluorocyclic ether, perfluorotripropylamine, cis-perfluorodecalin, transperfluorodecalin, perfluoro-1-methyl decalin, perfluoroisopentyltetrahydropyrane, perfluoro-N,N-dimethylcyclohexylamine, perfluoroperhydrophenanthrene, and perfluorotriamylamine.
To facilitate the separation of the heat transfer liquid from hydration water and reaction product water in the reactant exhaust stream, it is preferable that the heat transfer liquid be substantially immiscible with the water. If the reactant exhaust stream also comprises excess reactant, excess reactant may also be recirculated. It may be recirculated along with the recirculated heat transfer liquid, or separated. Reactant recirculation may increase fuel cell efficiency by improving reactant utilization. A fluid separator may be used to separate the heat transfer liquid from water and/or excess reactant in the exhaust stream.
A temperature sensor may be used to monitor the fuel cell temperature, directly or indirectly, and provide data for controlling, for example, the amount of heat transfer liquid that is introduced to the reactant fluid stream, and the extent to which it is cooled or heated before being introduced into the fuel cell electrode. In this way, the temperature within the fuel cell may be maintained within a pre-set temperature range. The temperature sensor is preferably located so that it contacts the fluids in the recirculation subsystem or the reactant fluid passages or the fuel cell separator plates. A reactant concentration sensor may be used to control the amounts and proportion of reactant and heat transfer liquid so that a desired reactant concentration is supplied to the fuel cell. The desired reactant concentration is generally a function of the power demand. The reactant fluid stream may be pressurized above atmospheric pressure to increase the amount of reactant that may be carried by the reactant fluid stream.
The method may further comprise using the heat transfer liquid to purge reaction product water and excess hydration water from the fuel cell when the fuel cell is shut down. The reactant flow passages and porous electrodes may be flushed or filled with the heat transfer liquid upon shut down. This can be especially beneficial if the heat transfer liquid has a freezing point substantially below that of water, and the fuel cell is to be exposed to low temperature conditions.
During normal operation, the heat exchanger removes heat from the heat transfer liquid to cool it before it is re-introduced to the fuel cell reactant fluid passages. In a preferred method, the heat exchanger controls the temperature of the heat transfer liquid between a reactant fluid passage outlet and the heat transfer liquid reservoir.
In one embodiment, the method comprises dissolving a gaseous reactant in the heat transfer liquid to produce a one-phase reactant fluid supply stream. The dissolution step generally comprises mixing the reactant into the heat transfer liquid to produce the reactant fluid stream. For example, the reactant may be bubbled into the heat transfer liquid using a sparger or another type of gas-liquid contactor.
However, in a particularly preferred method, the gaseous reactant is mixed with the heat transfer liquid and the reactant fluid stream is introduced to the first electrode as a two-phase fluid stream. The two fluid streams may be combined, for example, by pressurizing one of the fluid streams and injecting it into the other fluid stream. In this embodiment, the reactant fluid stream is preferably, but not necessarily, recirculated. In one embodiment of this method, the reactant fluid is combined with the heat transfer liquid inside a reactant fluid passage or manifold within a fuel cell assembly.
In yet another embodiment, a heat transfer liquid may be introduced into the reactant fluid passages of both the anode and the cathode. That is, both the oxidant and fuel fluid streams may comprise a heat transfer liquid. The same heat transfer liquid may be introduced into both oxidant and fuel fluid streams, or alternatively, different heat transfer liquids may be employed.
The method of the invention may be practiced by employing an electrochemical fuel cell power generation system, which comprises:
(a) a plurality of fuel cell assemblies arranged in a stack, wherein each of the plurality of fuel cell assemblies comprises:
an electrolyte interposed between a first electrode and a second electrode;
a quantity of electrocatalyst disposed at interfaces between the electrolyte and the first electrode and the second electrode;
a first reactant fluid passage adjacent the first electrode having an inlet and an outlet;
a second reactant fluid passage adjacent the second electrode having an inlet and an outlet;
(b) a first reactant supply subsystem comprising a first reactant supply manifold which fluidly connects a first reactant supply to the first reactant fluid passage;
(c) a second reactant supply subsystem comprising a second reactant supply manifold which fluidly connects a second reactant supply to the second reactant fluid passage;
(d) a heat transfer liquid supply subsystem comprising a reservoir which is fluidly connected to the first reactant fluid passage inlet, for introducing a heat transfer liquid into the first reactant fluid passage;
(e) a recirculation subsystem comprising a recirculation fluid passage fluidly connecting the first reactant fluid passage outlet to the first reactant fluid passage inlet; and
(f) a heat exchanger, disposed in the recirculation passage between the first reactant fluid passage outlet and the first reactant fluid passage outlet, for controlling the temperature of the heat transfer liquid.
The recirculation subsystem preferably directs the recirculated heat transfer liquid to the heat transfer liquid reservoir. Alternatively, the recirculated heat transfer liquid may by-pass the heat transfer liquid reservoir and join the first reactant fluid stream downstream of the heat transfer liquid reservoir.
The heat transfer liquid supply subsystem preferably further comprises a mixer for mixing the heat transfer liquid with the first reactant. The mixing may be accomplished by introducing the first reactant into the heat transfer liquid, but preferably, the heat transfer liquid is introduced into the first reactant. For example, the first reactant may be introduced into a heat transfer liquid reservoir or mixing tank, using a mixer such as a sparger, diffuser, or other type of gas-liquid contactor.
Alternatively, the heat transfer liquid may be injected into the first reactant using a mixer such as, for example, an injector or an atomizer. The injector nozzle is preferably oriented so it is aligned with the flow direction of the receiving fluid stream so that the injected fluid flows in the same general direction as the receiving fluid. The fuel cell power generation system may employ an injector that is external or internal to the fuel cell stack. For example, an internal injector may have an injector nozzle positioned at each of the first reactant fluid passage inlets. The injector may be supplied by a manifold that may be positioned within the fuel cell stack. Conveniently, the heat transfer liquid manifold may be positioned within a manifold that supplies the first reactant to the first reactant fluid passage. An advantage of this arrangement is that heat transfer liquid is supplied directly to each fuel cell assembly in the fuel cell stack and there is no opportunity for the heat transfer liquid to separate from the first reactant before being directed to the individual fuel cell assemblies. As shown by the above-described arrangements, the mixing location may be positioned anywhere between the heat transfer liquid reservoir and the entrance to the fuel cell fluid passages.
The heat transfer liquid reservoir may be a pressure vessel. If the first reactant is added to the heat transfer liquid in the reservoir, an advantage of pressurizing the reservoir is that at higher pressures more reactant can be dissolved in the heat transfer liquid. Alternatively, if the heat transfer liquid is added to the first reactant downstream of the reservoir, higher pressure is desirable for injecting the heat transfer liquid into the first reactant. The heat transfer supply subsystem may further comprise a pump for raising the pressure of the heat transfer liquid prior to injection.
A preferred recirculation subsystem further comprises a fluid separator for separating at least a portion of the heat transfer liquid from some or all of the other components of the first reactant exhaust stream and directing it to the heat transfer liquid supply subsystem. Accordingly, the separator is located downstream of the first reactant fluid passage outlet and upstream of the first reactant fluid passage inlet.
In a preferred embodiment of the fuel cell power generation system, a heat exchanger is located in the recirculation subsystem between the first reactant fluid passage outlets and the heat transfer liquid reservoir, so that the temperature of the heat transfer liquid may be controlled prior to recirculating the heat transfer liquid back to the reservoir. However, in an alternative embodiment, a heat exchanger may be located between the heat transfer liquid reservoir and the first reactant fluid passage inlets. In further alternative embodiments, a plurality of heat exchangers may be employed for controlling the temperature of the heat transfer liquid at more than one location in the fuel cell power generation system.
In another embodiment of the present fuel cell power generation system, the heat transfer liquid supply subsystem may be fluidly connected to both the first and second reactant fluid passages. An advantage of this arrangement is that heat transfer liquid flows in both of the first and second reactant fluid passages, increasing the direct contact between the heat transfer liquid and the fuel cell components. To recirculate the heat transfer liquid, the recirculation subsystem comprises a first recirculation fluid passage associated with the first reactant fluid passage and a second recirculation fluid passage associated with the second reactant fluid passage. In this embodiment, the recirculation subsystem preferably includes a fluid separator associated with each of the recirculation fluid passages to prevent interaction between any excess first and second reactants that may be present in the first and second exhaust fluid streams. In a preferred embodiment of this fuel cell power generation system, to ensure that the reactant streams do not mix in the heat transfer liquid reservoir, separate heat transfer liquid reservoirs are employed for receiving the recirculated oxidant and fuel exhaust fluid stream.
An advantage of the present fuel cell power generation system is that power density is increased by eliminating separate cooling layers. That is, more power can be produced by the present fuel cell stack compared to a conventional fuel cell stack with the same dimensions, or the present fuel cell stack can be made smaller than a conventional fuel cell stack and still produce the same amount of power. Eliminating separate cooling layers also reduces manufacturing steps and eliminates components associated with separate cooling layers such as fluid seals. Eliminating such components improves reliability by reducing the number of components that may potentially require maintenance service.
A further advantage is that, since pure water is not used as a coolant, the present fuel cell stack is more tolerant to exposure to cold temperatures. This tolerance to exposure to cold temperatures can be further improved by using a heat transfer liquid with a suitably low freezing point to purge product water and excess hydration water from the fuel cell upon shut down.